Charging surfaces for xerography



Sept. 1, 1964 K. M. OLIPHANT 3,

CHARGING SURFACES FOR XEROGRAPHY Filed Sept. 6. 1960 \NVENTOR KEJTHMERBDWH OuPHAN'r //%////7,M/M Lpw AT TO RN EYS United States Patent()3,147,415 CHARGING SURFACES FOR XERQGRAPHY Keith Meredith ()liphant,Westbournc Park, South Australia, Australia, assignor to ResearchLaboratories of Australia Limited, Adelaide, South Australia, AustraliaFiled Sept. 6, 1960, Ser. No. 54,187 Claims priority, applicationAustralia Sept. 9, 1959 6 Claims. (Cl. 317-262) This invention relatestoan improved method of charging photoconductor surfaces. In xerographyit is customary to carry a photoconductor substance in a film basesupported on a suitable backing either with or without a conductortherebetween and this material is used by first charging thephotoconductor substances in the dark and then bleeding away the chargeby the application of light to such areas where a pattern is to beformed.

The method of charging usually consists in subjecting the surface to acorona discharge of the required polarity, it being found that avariation in the charging occurs according to whether a positive ornegative polarity is used, this being dependent also of course on theparticular type of photoconductor materials used .and particularly thepolarity of the charges on the photoconductor particles prior tocharging.

Certain difficulties occur when using direct current for chargingphotoconductor surfaces, such as unequal charging and the presence ofelectrical flaws or inequalities on the final developed image due tocharging irregularities and paper grain and the like.

We have now found that improved charging results where AC. is used tocharge the photoconductor surface. From tests it is obvious that thephotoconductor surface itself is capable of acting as a rectifierprovided conditions are right and will only charge one way even thoughthe corona discharge used is of an alternating nature. Further, chargingis, as stated, more uniform.

The basis of our invention therefore is to use alternating current andto use an effective conductor for the charge by applying thephotoconduct-or surface to the conductor with a minimum of electricalresistivity, particular care being taken to ensure that there is nooxide film between the conductor and the film base which could form abarrier to electrical flow.

To understand the charging it can be assumed that when thephotoconductor substances are embedded in an insulating base such as analkyd resin, the photoconductor particles will have an inherent surfacecharge which exists on all particles suspended in an insulating medium.

The polarity of the charges varies with different materials, but whenusing a material such as zinc oxide, the surface charge will be of adefinite and fixed polarity.

When such a particle is then subjected to an electrical field whichchanges in polarity, there will be a tendency to reject charge of onepolarity but to accept the charge of the opposite polarity.

If therefore the flow of the alternating current is not materiallyrestricted, and can readily be conducted away by the conductor beneaththe photoconductor film, it is found that the photoconductor particleswill automatically reject the unwanted polarity but will be charged fromthe required polarity.

From the experiments conducted using alternating current charging forzinc oxide photoconductor films, it is obvious that a very uniform .andeffective charge is possible provided the lack of a barrier forelectrical flow between the film base and the conductor is assured.

It was found that if the conductor was applied to a backing such aspaper or glass or the like by vacuum coating, and if a condition wasmaintained which dis- 3,147,415 Patented Sept. 1, 1964 ice couraged theformation of an oxide coating on the conductor prior to the applicationof the film base, a very satisfactory photoconductor medium was givenwhich could be charged by means of alternating current and could then beprocessed in the normal manner, similarly to the methods used withphotoconductor surfaces charged by means of direct current.

It has also been found that the process can be used with materials wherethe backing has the photoconductor film applied directly to it withoutan intermediate conductor layer, provided during processing the backingis impregnated with a conductive liquid so that there is intimatecontact between the conducting liquid and the photoconductor film.

It seems clear from all of the experiments carried out, that thecritical factor is the avoidance of an electrical barrier between thephotoconductor surface and the conductor which bleeds away the unwantedsign charge, and it obviously also follows that when exposing such acharged surface to a light image, it is again advantageous to have theintimate contact between the film base and the conductor to ensure thatno undue barrier is formed for the bleeding away of the charges of thephotoconductor material.

To enable the invention to be fully appreciated, an embodiment thereofwill now be described with reference to the accompanying drawings inwhich:

FIG. 1 is a schematic view showing how the invention may be applied,using a photoconductor layer on a paper or like backing with aconductive substrate, and

FIG. 2 is a similar view but showing the photoconductor layer formeddirectly on a paper or like backing and placed on to a conducting plate,in this case the electrical continuity between the photoconductor layerand the conducting plate being ensured by treating the backing with aconductive liquid or the like.

Referring first to FIG. 1. A backing 1 of paper or similar material hasvacuum deposited on it a conductive substrate 2, such as a metal, andthis in turn carries a layer of photoconductive particles 3, such aszinc oxide, embedded in an insulating matrix 4, such as a resin. A highvoltage transformer 5 is connected between the conductive substrate 2and discharge means 6 by leads '7 and 8 respectively. The dischargemeans 6 include a series of points 9.

The points 9 are spaced some distance from the photoconductor layer 3-4,in actual practice a distance of perhaps half an inch for a dischargevoltage of 8,000 volts.

When alternating current is applied to the transformer 4, a coronadischarge takes place between the points 9 of the discharge means 6 andthe conductive substrate 2, but it is found that the photoconductorparticles 3 take a charge of only one polarity, consistent with theirown inherent surface charge, and in this way a very uniform andeffective potential is built up on the photoconductor layer. As thebacking 1 is below the conductive substrate, mottling of the surfacedoes not take place as where the charge must leak away through thebacking, and a much cleaner picture results. To allow electrical contactwith the conductive substrate, this may remain uncovered along an edgeby stopping the deposition of the photoconductor material short of suchedge during manufacture.

In the embodiment shown in FIG. 2 the arrangement is somewhat similar tothat shown in FIG. 1 and similar reference numerals are used forcorresponding members, but in this case the conductive substrate 2 isomitted and the photoconductor layer comprising the particles 3 and theinsulating matrix 4 are put down directly on to the paper or likebacking 1.

A metal plate 10 replaces the substrate 2, and the lead 7 is connectedto this plate 10.

The paper or like backing 1 is soaked in a conductive liquid, such as asaline solution, prior to charging, and a good electrically conductivepath thus exists between the photoconductor layer and the metal plate10, which again ensures that electrical flow between the discharge means5 and the metal plate can take freely, with the resultant rectificationeffect and uni-polar charging of the photoconductor layer as previouslydescribed herein.

As the paper or like backing is rendered conductive, mottling due topaper grain effects,aor lack of uniformity in the backing, is againavoided.

Any suitable paper is coated with a paint of the following compositionwhich forms the photoconductor layer:

Exampfe 1 Rhodene Resin L9/5O Trade Mark for a short oil linseed oilmodified alkyd resin, acid No. 25-35 marketed by Polymer CorporationPty. Ltd.,l lbs., Durham Microx Trade Mark for an indirect process leadfree zinc oxide, particle size 0.2-0.5 manufactured by Durham ChemicalLtd., zinc oxide30 lbs., toluene-1 /2 gallons; are blended and mixed ina ball mill for sixteen hours; after milling add:

Rhodene Resin L9/5010 lbs., lead napthenate 15%- 2 oz., cobaltnaphthenate 3%-l oz. The paint should be stored at this concentrationbut thinned prior to application to the paper. Thorough drying ispermitted to take place, and the paper is ready for use. When charging,an alternating potential of between 1000 to 8000 is applied, thedistance between the points 8 of the discharge means and the conductivesubstrate 2 on the metal plate 10 being such as to give a field strengthof about 8000 volts per inch.

Example 2 For zinc oxide substitute amorphous selenium powder.

Example 3 For zinc oxide substitute lead monoxide.

Example 4 For zinc oxide substitute cadmium oxide.

I claim:

1. In a xerographic process in which a layer of photoconductor materialis charged in the absence of illumination and subsequently exposed toproduce a charge pattern on the photoconductor, the charging methodcomprising: applying a layer of photoconductor material, comprising aphotoconductor selective to a given charge polarity, on a basecomprising an electrical insulator; establishing said layer ofphotoconductor material in electrical contact, at one surface thereof,with an electrically conductive backing; disposing the photoconductorlayer in spaced relation to a corona discharge means with the dischargemeans facing the surface of the photoconductor layer opposite theconductive backing; and applying a balanced high-voltage alternatingcurrent between the discharge means and the conductive backing to alforda balanced A.C. corona discharge therebetween and charge thephotoconductor layer to said given charge polarity, charges of oppositepolarity being discharged through said conductive backing.

2. The charging method of claim 1 in which the conductive backing isdirectly physically engaged with the photoconductor layer in the form ofa conductive substrate interposed between the photoconductor layer andthe base.

3. The charging method of claim 1 in which the conductive backing is inthe form of a conductive plate, the conductive plate is disposed incontact with the base, and the base is treated with a conductive mediumto establish electrical continuity between the photoconductor layer andthe conductive plate.

4. The charging method of claim 1 in which the electrically conductivebacking is afforded by a conductive metal substrate on the base, saidsubstrate being applied to the base by vacuum deposition before thephotoconductor layer is applied to the base.

5. The charging method of claim 3 in which the treatment employed toestablish electrical continuity-between the photoconductor layer and theconductive plate comprises Wetting the base with a conductive fluid.

' 6. A charging system for xerography in which a layer of photoconductormaterialis charged in the absence of illumination and subsequentlyexposed to produce a charge pattern on the photoconductor, comprising: aphotoconductive member comprising a layer of photoconductor material,selective to a given charge polarity, mounted on an insulator base; anelectrically conductive backing in electrical contact with the surfaceof said photoconductor layer facing said base; corona discharge means,disposed in spaced relation to the other surface of said photoconductor;and means for applying a balanced high-voltage alternating currentbetween said discharge means and said conductive backing to charge saidphotoconductor layer to said given charge polarity,

charges of opposite polarity being discharged through said conductivebacking.

References Cited in the file of this patent UNITED STATES PATENTS2,752,271 Walkup et al June 26, 1956 2,790,082 Gundlach Apr. 23, 19572,791,949 Simmons et a1 May 14, 1957

1. IN A XEROGRAPHIC PROCESS IN WHICH A LAYER OF PHOTOCONDUCTOR MATERIALIS CHARGED IN THE ABSENCE OF ILLUMINATION AND SUBSEQUENTLY EXPOSED TOPRODUCE A CHARGE PATTERN ON THE PHOTOCONDUCTOR, THE CHARGING METHODCOMPRISING: APPLYING A LAYER OF PHOTOCONDUCTOR MATERIAL, COMPRISING APHOTOCONDUCTOR SELECTIVE TO A GIVEN CHARGE POLARITY, ON A BASECOMPRISING AN ELECTRICAL INSULATOR; ESTABLISHING SAID LAYER OFPHOTOCONDUCTOR MATERIAL IN ELECTRICAL CONTACT, AT ONE SURFACE THEREOF,WITH AN ELECTRICALLY CONDUCTIVE BACKING; DISPOSING THE PHOTOCONDUCTORLAYER IN SPACED RELATION TO A CORONA DISCHARGE MEANS WITH THE DISCHARGEMEANS FACING THE SURFACE OF THE PHOTOCONDUCTOR LAYER OPPOSITE THECONDUCTIVE BACKING; AND APPLYING A BALANCED HIGH-VOLTAGE ALTERNATINGCURRENT BETWEEN THE DISCHARGE MEANS AND THE CONDUCTIVE BACKING TO AFFORDA BALANCED A.C. CORONA DISCHARGE THEREBETWEEN AND CHARGE THEPHOTOCONDUCTOR LAYER TO SAID GIVEN CHARGE POLARITY, CHARGES OF OPPOSITEPOLARITY BEING DISCHARGED THROUGH SAID CONDUCTIVE BACKING.